Controllable construction of porous high specific surface area carbon materials derived from walnut green peel for high-performance symmetric supercapacitors

Although biomass can be used to prepare carbon materials for supercapacitors, unmodified carbon materials have lower electrochemical properties. In addition, the complexity of biomass reactions during carbonization leads to irregular changes in specific surface area and pore structure. To this end, walnut green peel was used as carbon source and KHCO₃ was used as activator to prepare porous carbon materials with high surface area, which realized the precise regulation of specific surface area and pore structure of carbon materials. With the increase of KHCO₃ mass, specific surface area and pore structure of carbon materials showed a parabolic variation law. When the mass of KHCO₃ is 3 g, the synthesized WGP-3 is located at the apex of the parabola. WGP-3 exhibits the highest specific surface area and pore volume, as well as reasonable distribution of micropores and mesoporous pores. When the mass of KHCO₃ is 2 and 4 g, the synthesized WGP-2 and WGP-4 are located at the symmetric bottom of the parabola. The two samples have similar specific surface area and pore size distribution. The electrochemical test shows that WGP-3 has the best electrochemical performance. The current density of 1 A g⁻¹ exhibits a high specific capacitance of 227 F g⁻¹. Even when the current density increases five-fold, (5 A g⁻¹), the specific capacitance remains 199.5 F g⁻¹. Although the heteroatom content of different samples is different, the electrochemical properties of WGP-2 and WGP-4 are similar, indicating that the electrochemical properties of carbon materials mainly depend on the specific surface area and pore structure. A symmetric supercapacitor (SSC) assembled with WGP-3 has an energy density of 14.8 Wh kg⁻¹ at a power density of 350 W kg⁻¹. After 10,000 cycles, the capacitance retention rate is up to 100 %, showing good application value. This study provides a new method for the controlled synthesis of porous carbon materials with high specific surface area. The revealed relationship between structure and electrochemical properties will point to new direction for the design of high-performance carbon materials, thereby realizing the high-value utilization of waste biomass.